Underlay Beneath Shingles

(1)  Except as required in Sentence (2), when underlay is used beneath shingles, it shall be,

    (a)   asphalt-saturated sheathing paper weighing not less than 0.195 kg/m², or

    (b)   No. 15 plain or perforated asphalt-saturated felt.

 (2)  Underlay used beneath wood shingles shall be breather type.

Article Ontario Regulation 332/12 Building Code,   Information published by oncodes.ca for educational purposes only.

Felt paper roof underlay

Source : Canada Mortgage and Housing Corporation (CMHC)